Mill



R. P. FISHER April 3, 1962 MILL.

5 Sheets-Sheet 1 Filed Deo. 16, 1959 n! H O w Apll 3, 1952 R. P. FISHER 3,028,106

MILL

Filed Dec. 16, 1959 3 Sheets-Sheet 2 IN VEN TOR. /vmT P- P/wne A TTORIVEKS' April 3, 1962 Filed Dec. 16, 1959 R. P. FISHER 3,028,106

MILL

3 Sheets-Sheet 3 FIG. 5.

IN V EN TOR. OERT P. F75/IIR United States My invention relates to a new and improved mill for comminuting ores and other solid materials and to a new and improved method of comminuting solid materials.

According to one embodiment of my invention, I provide a mill which has a single unit which has a fixed casing or stator, and a rotor which is rotatable within said stator about a longitudinal horizontal axis. This casing has an inner cylindrical wall which is concentric with said axis of rotation. In this embodiment, the casing has a vertical front planar wall and a rear vertical planar wall. Said inner wall of said casing is conveniently `designated as a lateral inner wall. These planar walls are lateral and perpendicular to said axis of rotation.

The front lateral wall of the casing has a longitudinal inlet opening which is concentric with the axis of rotation of the rotor. The starting material, such as an ore, is thus fed longitudinally into the casing, together with air, substantially at the axis of rotation of the rotor.

The rotor has radial rotor blades which are fixed to a horizontal rotor shaft. These radial blades are inclined to each other to provide intermediate tapered rotor columns. These blades are rotated at suitable speed. The inner cylindrical face of the casing has inward stator bars extending radially inwardly from said inner face. The pieces of inwardly fed solid material are rapidly rotated by the impact of the rotor blades before said inwardly fed pieces fall by gravity to contact said inner cylindrical wall, so that said pieces of infed material are rapidly projected radially by centrifugal force against the inner cylindrical face of the stator and against the stator bars.

The radial impact of these projected pieces of solid infed material against the stator and against the stator bars shatters and comminutes the infed pieces. Also, at 'the inner cylindrical face of the casing, the infed pieces or the shattered infed pieces, are moved circumferentially by the rotor blades, to be impacted against the stator bars or to be sheared between the tip edges of the rotor blades and the stator bars.

In this embodiment, another important feature is to provide the single stator with a single, straight, vertical, cylindrical discharge pipe or conduit at the top of the stator. The rotor blades act as a centrifugal fan or pump, to draw a current of air into the casing through its inlet, together with the infed pieces of solid material, and to discharge ythe air in a vertical outlet -discharge current through said discharge pipe or conduit.

l regulate the velocity of said outlet discharge current of air.

if said velocity is low, any coarse piece of comminu'ted material which is drawn into said discharge pipe by said discharge current, drops by gravity back into the casing, to be further comminuted into smaller particle size. If the velocity of said outlet discharge current of air is high, said current will move relatively large particles of material out of said casing, into the external atmosphere, or into a suitable collector.

Hence, by selecting the velocity of said discharge current of air, the particle size of lthe comminuted material which is discharged from the casing can be selected.

The factors which control the flow of air in straight cylindrical pipes are well known. In general, if the pressure head is constant, the volume of the llowing 'Y l f f I 3,028,106 I aient 1C@ Patented Apr. s, 1962 current of air through a pipe varies inversely as the length of the straight pipe, and said volume also varles directly asV the inside diameter of the straight pipe. If there is a comparatively small and constant difference 1n air pressure between the two ends of the straight pipe, the volume of the flow of air through the straight pipe varies directly as the difference in pressure between the two ends of the pipe, and said volume of ow also varies directly according to the inside diameter of the pipe, and said volume of ow also varies inversely to the length of the pipe.

These factors and other factors are stated in pages 1287-1291 of the Fifth Edition (published in 1914) of Machinerys Handbook, published by the Industrial Press.

As one example, the air pressure at the outlet end of said straight discharge pipe may be standard atmospheric pressure of substantially 14.7 pounds per square inch, and the air pressure which is generated by the centrifugal pump action of the rotor blades at the inlet end of said discharge pipe may be 17.5 pounds per square inch, corresponding to a pressure head of 2.8 pounds per square inch.

Hence the rate of the discharge of air through the outlet pipe under a constant pressure head can be regulated by selecting an outlet pipe of selected length and of selected internal diameter, or by varying the length of the outlet pipe by making it of a plurality of adjustable sections, or by varying the effective internal diameter of said outlet pipe by providing said pipe with the wellknown pivoted Ibutterfly valve. By increasing the effective length of the adjustable pipe, or by decreasing its effective diameter by means of said valve, the volume and the velocity of the outlet air current can be decreased, while rotating the rotor at constant speed.

Thus, the speed of rotation of the rotor is selected, in order to provide the necessary mechanical force for shattering and/or `shearing the infed solid material, and for other purposes.

At said selected constant speed of rotation, the rotor will generate a respective constant internal air pressure within the casing and at the inlet end of the straight discharge pipe.

The particle size of the comminuted solid material which is discharged from the casing can be selected, as one example, by selecting the height of the vertical and straight non-adjustable discharge pipe. If a long discharge pipe is selected, the discharge air current through said discharge pipe will be of low volume and low velocity, so that gravity will prevent coarse comminuted particles from being carried out of said casing thorugh said non-adjustable and straight and vertical discharge pipe.

Instead of having a rigid, one-piece discharge pipe, said discharge pipe can be made of two or more telescopic and adjustably connected sections, in order to increase or decrease the `effective length of said discharge pipe.

Instead of regulating the velocity of flow of the discharge air current by either selecting or adjusting the length of the vertical discharge pipe, I can select or adjust the effective diameter of the inner cylindrical surface of the discharge pipe. The usual butterfly valve can be used for regula-ting the effective diameter of the inner cylindrical surface of the discharge pipe. As is well known, -said butterfly valve is a disc which is turnable around a diametral shaft which is turnably connected to the wall of the pipe. This disc can be manually adjusted to obstruct a selected part of the internal area of the cross-section of the discharge pipe.

I thus provide a very simple and efficient means and y 3 method for regulating the particle size of the discharged comminuted material.

I provide a very simple mill of small size, low cost, and high capacity.

I also provide a mill which can comminute relatively hard materials, with low abrasion of the rotor blades and the inner face of the casing.

Other objects of my invention are set forth in the annexed drawings and in the following description which illustrates preferred embodiments of my invention.

FIG. l is a longitudinal side elevation of one embodiment of the apparatus.

FIG. 2 is an end elevation of FIG. 1.

FIG. 3 is a longitudinal vertical section, partially in elevation.

FIG. 4 is `an end elevation of the mill at the front thereof, with the front cover of the casing removed. The drawings illustrate a full-sized commercial embodiment of the improved mill.

. FIG. 5 illustrates a variation of the first embodiment.

The apparatus is supported on the top horizontal flanges of Z-shaped angle irons 2 and 3 which have vertical, planar and longitudinally disposed webs B. The bottom lateral flanges of these Z-shaped angle-irons are fixed to any suitable support. These Z-shaped angle-irons have top lateral flanges. The casing S of the stator is fixed to the abutting flanges of respective angle-irons 31, which have bottom lateral anges which abut the top lateral flanges of the Z-shaped angle irons Z and 3. The abutting dianges of the angle-irons 31 and of the Z-shaped angle irons 2 and 3 are releasably fixed to each other by vertical bolts 32 and nuts 33.

Lateral and horizontal bars 4 and S are fixed to the top horizontal anges of the longitudinally `disposed Z- shaped angle-irons 2 and 3.

The bearings 6 and 7 of a shaft 8 are fixed to said lateral bars 4 and 5. This shaft 8 is the rotor shaft. It is horizontally and longitudinally `disposed in this embodiment. This shaft `8 is rotated around its longitudinal and horizontal axis, at a selected speed of rotation, by means of a conventional variable speed motor or drive 9, which is connected to shaft 8 by the usual coupling 1), which may be of the flexible type.

This longitudinal drive shaft 8 extends through the rear planar and vertical lateral wall 11 of the cylindrical casing or stator S. This rear wall 11 is detachably fixed to the rear annular wall of the stator S by means of screws 12. As shown in FIG. 3, the shaft 8 has a close but easy rotary fit Within a longitudinal axial hole or bore in the rear wall 11 of said stator S. This casing S has the shape of a cylinder which is concentric with the longitudinal axis of the shaft 8. The shaft 8 has a threaded part 8a Within the stator S. Said threaded part 8a of shaft 8 is threaded into a tapped bore of the rotor R of the mill, so that said rotor R is rotated in unison with the shaft 8.

As shown in FIG. 3, the inner threaded end-part 8a of the shaft 8 is of smaller diameter than the main body of said shaft 8, thus providing the shaft 8 with an annular shoulder 8s which abuts the rear wall of the hub of rotor R, in the assembled device. In addition to the detachable connection between the rotor R of the mill and the shaft 8 which is provided by the threaded part 8a of said shaft 8, the usual removable key 114 may be provided for securing an additional detachable connection between the shaft 8 and the rotor R.

This rotor R has a rear cylindrical body 15, in front of which said rotor R has a frusto-conical shape which has a truste-conical face 16. This frusto-conical front face 16 terminates at a vertical and lateral planar wall or surface 17 which has a circular periphery and which is in a plane which is perpendicular to the longitudinal axis of the drive shaft 8.

Four identical rotor blades 18 are fixed to the frustoconical front face 16 of the rotor R. These rotor blades 18 are of uniform thickness as illustrated in FIG. 4. Said blades 18 are equally angularly spaced by respective angles of ninety degrees to provide four tapered rotorcolumns. Each said rotor blade 18 has a straight and horizontal peripheral longitudinal edge or face 19. Each of said rotor blades 18 has a straight inner edge or face 21% which is parallel to the axis of shaft 8, and each of said rotor blades 18 has a straight front face or edge 20a which is perpendicular to the axis of yshaft 8. Each of said rotor blades 1S therefore has a trapezoidal shape.

FIGS. 1-4 are to scale and reference is made thereto for further disclosure. The scale for FIG. 1 and FIG. 2 is shown in FIG. 1 and the scale for FIG. 3 and FIG. 4 is shown in FIG. 4.

Dimensions referred to herein are given in inches and millimeters. A conversion factor of 1"=25.4 mm. was employed.

In this full-sized Working unit which is illustrated herein, the diameter of the rear planar and vertical wall 11 of the stator S is twelve inches, or substantially 305 millimeters. Therefore, the other actual dimensions of this full-sized unit can be easily determined from an inspection of FIGS. 3 and 4, in which the illustrated diameter of the rear wall 11 is six inches or substantially 153 millimeters, since these figures are to one-half of actual size.

In said full-sized working unit, the clearance between the outer edges or tip edges 19 of the rotor blades 18 and t-he inner cylindrical face SA of the fixed casing or stator S, is one-half inch or substantially 12.7 millimeters. This dimension rof one-half inch is also the clearance between the cylindrical edge of body 15 of the rotor R and the inner cylindrical face SA of the stator S, in said fullsized working unit. The stator S is provided with a front wall 21 which is detachably connected to the front face of the stator S by means of screws 22.

The front planar and vertical wall 21 of the stator S is provided with a longitudinal inlet hole or bore 23 to which the flange 25 of an inlet hopper 24 is detachably fixed by means of screws 26. As illustrated in FIG. 3, the horizontal longitudinal axis of the inlet opening 23 is coincident with the axis of the shaft 8, and the diameter of said inlet opening 23 is substantially equal to the diarnetral space between the diametrally opposed inner edges 20 of diametrically opposed rotor blades 18. A mixture of air and solid material can thus be fed into the stator S through said inlet hopper 24 and the inlet opening 23.

The stator S is provided at its top with an outlet passage or conduit 27, which is vertically disposed and connected to the zenith of the connecting wall or shell S. The outer wall of outlet passage or conduit 2.7 is fixed to the shell S by weldingvW. In this example, the outlet passage 27 is a Vertical cylinder.

As previously noted, the height of the cylindrical outlet passage 27 and the internal diameter are important factors. In the actual commercial embodiment, the member '27 is merely a cylindrical stud to which a longer and vertical cylindrical stack is attached. The total effective height of this stack and of the stub member 27 is five feet in the actual embodiment. This stack is not shown in the drawings. Its internal wall is cylindrical and of substantially the same diameter as the internal cylindrical wall of stud member 27.

In this embodiment, five identical and longitudinal and radially disposed stator bars 28 are held detachably xed to the stator S. In this example, the stator bars 28 have rectangular cross-sections. The outer part of each bar 28 vfits within a radial recess 29 in the inner lateral and cylindrical wall SA of stator S.

As shown in `FIG. 3, the annular peripheries of the front and rear casing walls 21 and 11 are of reduced thickness in comparison with the main bodies of said walls 21 and 11, in order to provide each said wall 21 and 11 with a cylindrical shoulder 30. The longitudinally opposed ends of the stator bars 28 abut the respective `shoulders 30, and these ybars 2S are releasably clamped into fixed position relative to stator S at walls 21 and 11, by means of the clamping screws 22 and 12 and said shoulders 30 of walls 21 and i1. These longitudinal stator bars 2S wear or abrade much more rapi ly than the rotor blades 18 and the inner wall SA of casing S. By providing easily detachable front and rear walls 21 and 11, it is possible easily to remove and to replace any abraded stator bar or bars Z3. The planar inner exposed faces of bars 28 are tangential to circle 30.

In this embodiment, the radial spacing in the full-sized operative unit, between the exposed inner faces of the bars 28, as defined by circle 30, and the straight outer edges 19 of the blades i8 is 0.25 inch or about 6.35 millimeters. The radial spacing between the inner exposed faces of said bars 28, as defined by circle 30, and the inner cylindrical face SA of shell S is also 0.25 inch or about 6.35 millimeters in this embodiment of the fullsized operative unit.

In the full-sized commercial embodiment illustrated herein, the radial distance of each tip edge 19 from the axis of rotation of shaft 8 is substantially 4.75 inches or substantially 121 millimeters, which corresponds to a circle which `has a circumference whose length is close to thirty inches.

Hence, if the shaft S is rotated at ten thousand revolutions per minute, the edges 19 will move in a circular path at the rate of substantially twenty-five thousand feet per minute.

In the tests later mentioned herein, the shaft 8 was revolved at 10,000 revolutions per minute for best results.

If the illustrated apparatus is used as a centrifugal fan while shaft 8 is rotated at 10,000 revolutions per minute, without feeding any solid material into the casing, and if the external atmosphere is at standard atmospheric pressure of substantially 14.7 pounds per square inch, said apparatus will generate a constant air pressure within the casing S of substantially 17.5 pounds per square inch, subject to an error or variation of twenty percent. In said illustrated commercial unit, this corresponds to an outlet air current which has a velocity of substantially 6,000 feet per minute through the stub pipe 27 and its attached outlet stack, which have a combined total height of five feet `and are of uniform internal cross-section.

The method of calculation applicable to centrifugal fans and ducts is stated in Elementary Fluid Mechanics by I. K. Vennard, which was published in 1951 by lohn Wiley & Sons, Inc.

Although best results in this embodiment were secured when shaft S was revolved at ten thousand revolutions per minute, good working results were secured when shaft 8 was rotated at a speed as low as three thousand revolutions per minute.

Tests have been made with gravel which had a high content of quartz; and with chips or shavings of cast iron.

Test No. 1

In making this test, the embodiment of FIGS. l-4 was used, and shaft 8 was rotated at six thousand revolutions per minute. The external atmosphere was at a pressure of substantially 14.7 pounds per square inch.

In making this test, the test material was said gravel. The average input size of the pieces of gravel was 0.25 inch or about 6.35 millimeters.

As previously noted, the radial clearance between the tip edges 19 of the rotor blades 18 andthe inner cylindrical wall SA of stator S is one-half inch in this embodiment, and the radial clearance between said tip edges 19 and the exposed inner faces of stator bars 28 is 0.25 inch. rfhe comminuted gravel which was discharged from the vertical pipe Z7 of this embodiment was yanalyzed for particle size by passing it successively through Sieves Nos. 28, 60, 100, 170, 250, 325. A comminuted specimen which weighed 2.75 pounds or 76 ounces was thus analyzed for particle size.

6 These sieves are in the U.S. Sieve Series. The characteristics of the sieves of this series are well known and many of the sieves in this series are described in page 643 of the 1934 edition of Handbook of Chemistry by Lange, published by Handbook Publishers, Inc. The characteristics of these Sieves Nos. 28, 60, 100,

170, 250, 325 are as follows:

Approximate Sieve Meshes per Opening Siero No. Lineal Inch Inch Millimeters The No. 28 screen retained 3.5 ounces of this comminuted specimen or about 5% by weight thereof. This fraction of about 5% by weight therefore had a particle size greater than 0.5800 millimeter, and about 95% by weight of the comminuted material had a particle size less than 0.5800 millimeter.

The No. 60 screen retained 10.5 ounces of this specimen or about 13% thereof.

The No. screen retained 15.5 ounces of this specimen or about 20 percent thereof.

ri`he No. screen retained 7.5 ounces of this specimen or about one percent thereof.

The No. 250 screen retained 3.5 ounces of this specimen or about 4% thereof.

The No. 325 screen retained 1.5 ounces of this specimen or about 2% thereof. A quantity of 34 ounces of this batch passed through the No. 325 screen, so that thirty-four ounces of this batch had a particle size smaller than `0.044 millimeter.

Hence, the average particle size of the comminuted output was much smaller than the radiali spacing of Substantially 6.35 millimeters between the outer peripheral edges 19 of the rotor blades 18 and the exposed inner faces of the stator bars 29.

These are some main effects which produce this improved result.

One effect is that of the impact of the infed particles against the inner wall SA of the stator or against the exposed parts of the bars 28. This impact shatters the frangible pieces of infed gravel. This impact is produced by the centrifugal force of the rotor blades 1S. The infed particles are moved both radially and circumferentially by the rotor blades 18.

Another effect is the shearing of the particles against the bars 2S.

Another effect is the grinding of the particles against each other in certain areas, such as the areas immed iately at and anterior the bars 2S in the direction of rotation of the rotor.

Another effect is the selected discharge through the vertical discharge pipe at the top of the casing. This discharge is controlled in this embodiment by the selected constant .height and diameter ofthe pipe outlet 27 and its associated stack, and the selected constant velocity of the rotor 5.

Also, it is important to have more stator bars 28- than rotor blades 18. In this embodiment, there is an angular spacing of ninety degrees between adjacent rotor blades 1S, and there is an angular spacing of seventy-two degrees between adjacent stator bars 28.

These stator bars 28 produce eddies of air in the tapered ninety-degree rotor columns which are provided between adjacent rotor blades 18. The air is discharged radially outwardly from each said tapered rotor column, together with the solid material in each saidy tapered rotor column, in a pathfwhich has a radial component and a circular component;

Assuming that only air is fed into the casing S due to the suction effect of the rotor blades 13, the air is discharged from each tapered rotor column to produce a vein of air at the respective part of inner face SA of stator S, and this vein of air is moved circumferentially along the inner face SA at high velocity and turbulent flow. Each vein of air is impacted against -the respective radial exposed face of the respective stator bar 23. This inwardly detlects each circumferentially moving vein of air into the respective tapered rotor column, so that turbulence of the air in each tapered rotor column is produced.

A high speed of rota-tion of the rotor thus produces suitable turbulence of air within each tapered rotor column, so that the pieces of solid material which are fed into each tapered rotor column are impacted and rubbed against each other in each ytapered rotor column, before being discharged from the tapered rotor column to irnpact against the inner wall SA of rotor S, or against the inner exposed faces of stator bars 28 which are defined by circle 3G.

It is noted that the outlet conduit 27 is located in a part of casing S which is circumferentially between adjacent stator bars 28. While the stator bars 28 have been shown as having square cross-sections, said stator bars 23 may have cylindrical cross-sections, and each cylindrical bar 28 may be rotatable around its longitudinal axis relative to casing S.

Test With Cast Iron The shaft 8 was rotated at 10,000 revolutions per minute.

The comminuted output pieces of cast iron were tested by sieves Nos. 28, 60, 100, 170, 250 `and 325 in said series. The percentages stated below are by weight.

1.8% was retained by No. 28. 45.8% was retained by No. 60. 21.1% was retainedby No. 100. 14.7% was retained by No. 170. 7.4% was retained by No. 250. 6.4% was retained by No. 325. 2.8% passed through No. 325.

The average input size of the iron pieces was substantially 1A@ inch, or substantially 1.6 millimeters.

Increasing the rotor speed results in an increase of the impact shattering eiect, but it simultaneously results in a greater velocity of outlet air current, which tends to carry the coarser particles out of the mill through outlet 27 and its stack.

In the mill of this embodiment, the casing and the rotor blades and the rotor were made of low carbon steel. The rotor disc 15 and the rotor -blades 18 were made of S.A.E. No. 02 alloy, hardened to Rockwell C-55 and the bars 28 were made of the same material. The casing was unhardened low carbon steel. The capacity of the mill in grinding the gravel was one-third of a pound per second.

FIG. shows an outlet cylindrical pipe which consists of a plurality of telescopic and adjustable sections 27A and 27B and the stub pipe 27. By increasing the total effective height of this adjustable pipe 2.7-27A-27B, the velocity of the outlet air current can be retarded, for a selected constant velocity of rotation of the rotor shaft 8. The sections 27A and 27B are held in adjusted position by releasable clamping screws 27s.

FIG. 5 also shows a modification in which the outlet stub pipe 27-27A-27B has a butterfly valve 27V, of the usual disc shape, which is xed to a horizontal shaft 27a, which can be turned by hand, in order to regulate the position of butterfly valve 27V. By moving said butterfly valve 27V to obstruct the pipe 2.7 to a selected extent, this regulates the effective inner diameter of the combined outlet pipe 2.7-27A-27B so that the iow of air and comminuted material out of pipe 27 can be regulated.

8 Either of the sections 27A or 27B may be provided with means for controlling its effective inner diameter. The invention is not limited to the vertical location of the outlet conduit 27.

It is suiiicient if said location is sufficiently or substantially vertical, so that the force of gravity will return coarse particles which enter the discharge conduit, back to the casing S for further comminution. Also, the inner wall of the outlet conduit need not be cylindrical. The outlet conduit may have an internal cross-section which may be elliptical or oval or of any other shape. Also, the outlet conduit need not be straight, as long as it has a leg or branch in which the force of gravity will return to the casings pieces of material which are larger than the selected comminuted size.

By selecting suitable high rotor speed, air waves are generated in each rotor column, having a frequency above the average audible limit of 9,000' cycles per second, and even above the supersonic frequency. Hence, during the operation of the mill, each of the rotor columns is continuously provided with air vortices and compressural waves. This results in comminuting the frangible solid material to some extent, before the frangible solid material is ejected from the outlet ends of the vortex columns.

The stator bars result in a protective layer of compressed air which is maintained at the inner wall SA of the stator.

Tests have been made in comminuting crude, metalliferons ores such as the ores of copper suliide, zirconium oxide, and ores of native gold, native platinum, native silver, native copper and other native metals. When these crude ores are comminuted, they contain valueless gangue impurities as exempliiied by quartz, silica, felspar and other impurities.

When these crude ores have been comminuted by the improved apparatus and process, this was done without any preliminary concentration or other treatment of the said crude ores, save that in some cases, these crude ores were crushed. When the comminuted crude ores were examined under the microscope, such examination showed that the valuable comminuted particles of the valuable metalliferous material, including the particles of native metal, were pure and were not contaminated by the valueless gangue material.

Also, the comminuted particles of valuable metalliferous material had sharp and clean and sharply-defined edges, whereas ordinary mechanical grinding or milling action would result in particles having round and blunted edges.

'Ihe initial particle size of the infed material which is fed into the mill may exceed the clearance between the tips of the rotor blades and the exposed tips of the stator blades.

In addition to working or comminuting a crude ore which contains either a metallic compound or a native metal, the material which is worked may be a liquid or may be a mixture of liquids.

The vortices are generated at the exposed tips of iixed stator bars, and these vortices are transmitted into the rotating rotor columns in an inward direction which is opposed to the outlet feeding direction.

The pieces of infed solid material may be substantially comminuted to the final small size within the respective rotor column, by means of the kinetic energy of the vortices.

This application is a continuation-in-part of my application Serial No. 684,677, tiled on September 18, 1957, in the United States Patent Office.

The invention is further disclosed and is dened in the appended claims.

I claim:

l. In a mill, a combination which includes a rotor which is iixed to a rotor shaft which has a horizontal axis of rotation, a stator in which said rotor is located,

said stator having a front stator -wall and a rear stator wall and a connecting stator wail between said front and rear stator walls, said rotor having a plurality of rotor blades Within said casing, said rotor blades being radial relative to said rotor shaft and being inclined to each other to provide a tapered rotor column between each pair of adjacent rotor blades, said rear wall having an opening through which said rotor shaft extends rotatably, said front Wall having an inlet opening aligned with the axis of said rotor shaft for admitting air and material to be comminuted, said connecting stator wall having a substantially vertical outlet conduit `connected to its zenith, said conduit being the only outlet for material comminuted in said casing, said connecting stator wall having stator bars detachably connected thereto and extending inwardly from the inner face of said inner stator Wall, said stator bars being disposed throughout the extent of the connecting stator Wall, the inner ends of said stator bars being spaced from the outer ends of said rotor blades.

2. In a mill, a combination according to claim 1, said stator bars being spaced apart by 72, and the rotor blades being spaced apart by 90.

3. In a mill, a combination according to claim l, said outlet conduit being an adjustable length conduit and having a valve operatively positioned therein, whereby the extent to which material is comminuted in the mill can be controlled.

References Cited in the le of this patent UNITED STATES PATENTS 1,669,239 Grindle May 8, 1928 1,683,500 Thordarson Sept. 4, 1928 1,790,967 Auerbach Feb. 3, 1931 1,945,054 MacGregor Jan. 30, 1934 1,988,743 MacKenzie lan. 22, 1935 2,322,306 McLaren June 22, 1943 2,392,958 Tice Ian. 15, 1946 2,414,361 Cowles Jan. 14, 1947 2,538,992 Trask Jan. 23, 1951 2,634,915 Fisher et al Apr. 14, 1953 2,679,933 Lockhart June 12, 1954 2,704,635 Trost Mar. 22, 1955 2,750,120l Pallman June 12, 1956 2,846,150 Work Aug. 5, 1958 2,861,880 Hammon Nov. 23, 1958 FOREIGN PATENTS 858,623 Germany Dec. 8, 1952 

